The invention relates to devices, such as flight simulators, where the position (angular or linear) of a movable element relative to a fixed element must be controlled very accurately. The invention relates more specifically to the position measurement that is used in the servo control of such devices.
A flight simulator provides a platform supported in a gimballed arrangement, so that the platform may be rotated about one or more axes. A control system for rotation about one of the axes typically includes an actuator such as a torquer, a position sensor such as a resolver, a mechanism for generating position commands, and a closed loop servo for causing the measured position to track the commanded position.
U.S. Pat. Nos. 4,253,051 and 4,988,936, hereby incorporated by reference, describe portions of a representative prior art system. In brief, the position sensor typically includes a stator, a rotor inductively coupled to the stator, and circuitry for exciting the stator and detecting the induced signal in the rotor. The relative phase between the stator and rotor signals is representative of the relative displacement between the stator and rotor.
A known way of measuring the phase difference is to generate pulses at a frequency that is a fixed factor multiple of the rotor output signal frequency. The number of pulses occurring between corresponding points in the stator and rotor signal cycles, divided by the fixed factor, gives the fraction of a cycle that the two signals are out of phase. The frequency multiplication may be accomplished by a digitally closed phase locked loop (PLL). The PLL includes, among other things, a frequency divider corresponding to the fixed factor multiple to be achieved, and a counter whose output represents the relative position.
It is known in the art to provide separate coarse and fine measurements of the relative position. Each requires a separate resolver and PLL. The coarse number is capable of defining the position within the entire expected range, while the fine number defines positions within a range that is much narrower. For example, a two-pole-resolver generates a 360.degree. phase shift between the stator excitation signal and the rotor output signal for every 360.degree. of relative mechanical rotation, and is suitable for extracting the coarse number. A 720-pole resolver produces an electrical phase shift of 360.degree. for every 1.degree. of relative mechanical rotation, and may be used to extract the fine number. The coarse and fine numbers are used to generate coarse and fine position errors for the servo.
In the prior art system the coarse and fine numbers are BCD entities having ranges of 000.0 to 359.9 and 0.0000 to 0.9999, respectively. The fine number's tenths digit is the more accurate one, so to account for possible misalignment as the rotors are rotated over 360 degrees, the tenths digit of the coarse number is offset by -0.5 degrees, and the coarse number is latched only when it has caught up to the tenths digit of the fine number. One characteristic of the prior art position measurement system is that the coarse and fine numbers (which provide the computed position) are available only once on each cycle of the resolver excitation.